1. Field of the Invention
The present invention relates to the manufacture of plastic bags or packages having at least two plies of thermoplastic sheeting, both plies having closure strips included on their facing inner edges so as to form a zipper-like closure for the openings of the bags or packages made therefrom. The plies may be formed from two separate sheets or from a single sheet folded over. More specifically, in accordance with the present invention, the closure strips are extruded from a synthetic polymeric resin material including a blowing agent, which foams the closure strip to a desired and advantageous degree.
2. Description of the Prior Art
The present invention relates to improvements in the package-making art and may be practiced, without limitation, in the manufacture of thermoplastic bags and trays of the kind that may be used for various consumer products, but which are particularly useful for food products which must be kept in moisture- and air-tight packages, free from leakage until opened for access to the product contents, which packages are then reclosable by zipper means to protect any remainder of the product therein.
The indicated art is fairly well-developed, but nevertheless is still susceptible of improvements contributing to increased efficiency and cost effectiveness.
The zipper means alluded to in the above are most commonly male and female interlocking zipper profiles extruded from low density polyethylene (LDPE). These zipper profiles are usually attached to the polymeric sheet material, from which bags or packages are being produced, either during a separate bag formation operation or in conjunction with the bag filling operation on a form-fill-and seal (FFS) machine. The former providing empty bags to be separately filled while the later provides filled bags.
While FFS machines are of several specific designs, all comprise a filling tube, about which the bags or packages are formed and through which premeasured amounts of a consumer product, such as a food material, may drop as individual bags or packages are being produced in a sequential fashion. On the FFS machine, polymeric sheet material is directed toward and around the filling tube by means of a forming collar, the two lateral edges thereof being brought together to form a fin extending outward from and longitudinally along the filling tube. The male and female interlocking zipper profiles may be directed between the two lateral edges and are joined or heat-sealed thereto to form the facing inner portions of the reclosable bag opening.
As bags or packages are being produced and filled in a sequential fashion on an FFS machine, seals are made transversely across the polymeric sheet material and interlocked male and female zipper profiles to form the side seals of individual packages. Because of the bulk of the zipper profiles relative to that of the polymeric sheet material, the zipper profiles require some pre-seal treatment in the location where a side seal is to be made, so that the side seal may be complete and not permit any leakage of the contents of the package. Such pre-seal treatment includes the use of heat stompers (hot anvils which crush the zipper profiles at the location of the side seal to be subsequently made), ultrasonic welders (similar to heat stompers but employing ultrasonic energy), hole punchers (which remove a portion of the zipper profile where the side seal is to be applied), and milling cutters (which abrade away a portion of the zipper profile at the side seal location). The pre-seal treatment permits the cross sealing equipment to make a reasonable seal. It goes without saying that the pre-seal treatment of the zipper profiles is an extra manufacturing step which, unfortunately, does not entirely eliminate the occurrence of leaking packages, or "leakers".
The present invention is intended to reduce the extent of the pre-seal treatment required where a side seal is to be cut across a zipper profile, to reduce the occurrence of "leakers", and to improve the air tightness of the lock.